This invention relates to a fluidized bed reactor and a method of operating same and, more particularly, to such a reactor and method in which a flue gas by-pass system is provided for channeling a portion of flue gases to a heat recovery area.
Fluidized bed reactors, such as qualifiers, steam generators, combustors, and the like are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an absorbent for the sulfur generated as a result of combustion of the coal, to fluidize the bed and promote the combustion of the fuel at a relatively low temperature. The entrained particulate solids are separated externally of the bed and recycled back into the bed. The heat produced by the fluidized bed is utilized in various applications such as the generation of steam, which results in an attractive combination of high heat release, high sulfur absorbtion, low nitrogen oxides, emissions and fuel flexibility.
The most typical fluidized bed reactor is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well defined, or discrete, upper surface.
Other types of fluidized bed reactors utilize a "circulating" fluidized bed in which the fluidized bed density is well below that of a typical bubbling fluidized bed, the air velocity is greater than that of a bubbling bed and the flue gases passing through the bed entrain a substantial amount of particulate solids and are substantially saturated therewith.
Also, circulating fluidized beds are characterized by relatively high solids recycling which makes them insensitive to fuel heat release patterns, thus minimizing temperature variations, and therefore stabilizing the emissions at a low level. The high solids recycling improves the efficiency of the mechanical device used to separate the gas from the solids for solids recycle, and the resulting increase in sulfur absorbent and fuel residence times reduces the absorbent fuel consumption. However, several problems do exist in connection with these types of fluidized bed reactors, and more particularly, those of the circulating type. For example, a circulating fluidized bed reactor typically must be designed to function at near isothermal conditions within a fairly precise and narrow range of temperatures for maximum sulfur capture and solids stabilization. When operating at a relatively low load, it is very difficult to maintain these temperature conditions since the flue gas temperature leaving the furnace section and entering the heat recovery area tends to drop significantly. The furnace exit flue gases become cooled to the point where the efficiency of the downstream convection heat exchange surfaces suffer and thus more elaborate or extra surfaces are required. A superheater so modified, in addition to requiring larger and more expensive superheat and/or reheat surfacing, also produces undesirably large attemperation requirements at full load. In order to maintain acceptable temperatures during operation of these modified superheaters, recycle solid stream temperature and flow control techniques, variable external heat exchangers and other expensive means of temperature control have also been employed. However, the addition of these components also adds to the cost and complexity of the system.